The long-term objectives of these studies are to determine the mechanisms effecting the regulation of poxvirus gene expression. This work will include studies on aspects of transcriptional control, translational control, mRNA stability, and the effects of viral gene products on the host cell and the host animal. Specifically, the following will be investigated: 1. The generation of the 5'-poly(A) leaders of late mRNAs, and the effect of these leader sequences on translational efficiency and mRNA stability. 2. The generation of the defined 3'-ends of the late mRNAs of certain strongly-expressed genes; the effect of these 3'-end formation on downstream transcription units. 3. The regulation of expression of the gene encoding the second-largest sub-unit of the viral DNA- dependent RNA polymerase. The functions of this sub-unit will also be examined. 4. The expression of the 38K gene at intermediate times during the viral replication cycle. The role of its gene product in the inhibition of proteinases, and its effect on the virus-host interaction will also be investigated. Standard biochemical and genetic procedures, including site-directed mutagenesis procedures and a novel genetic complementation system, will be used to achieve these goals. Advantage will be taken of the unique properties of the poxviruses. These studies will contribute to our understanding of molecular events involved in the regulation of gene expression in eukaryotes in general, and in poxviruses in particular. Studies on the viral DNA-dependent RNA polymerase, which is structurally similar to the eukaryotic RNA polymerases. Studies on the 38K gene's product should advance our understanding of virus-host interactions resulting in hemorrhage or leukocyte accumulation at the site of the infection. In addition, these studies will help us to develop more efficient poxvirus expression vectors, and contribute to the development of safe and effective poxvirus-derived vaccines.